1. Field of the Invention
The present invention relates to clock and data recovery, and more particularly to an apparatus for clock and data recovery.
2. Description of Related Art
An electronic high-speed transmission system, such as a wired communication system or a wireless communication system, can be modeled as a transmission system that includes a transmitter, a receiver and a transmission channel. In such a transmission system, the transmitter generates a transmission signal according to a clock signal, and transmits the transmission signal to the receiver through the transmission channel.
However, characteristics of the transmission channel impacts quality of signal transmission. For example, when the transmitter transmits a transmission signal of a square waveform to the receiver, the receiver will receive a waveform with slowly rising and falling edges, instead of a square waveform with sharp rising and falling edges. In the waveform received by the receiver, the slowly rising portion is referred to as a pre-cursor, the peak of slowly rising forms a main cursor, and the slowly falling portion after the peak is referred to as a post-cursor. Such signal distortion will cause ISI (inter-symbol interference) and degrade the quality of signal transmission, e.g., increase the bit error rate (BER).
To compensate impacts of the transmission channel, the receiver is equipped with equalization mechanism. For example, when the receiver receives the incoming signal from the channel, the receiver equalizes the received signal in response to equalizer coefficients and uses a clock and data recovery (CDR) circuit to generate a recovered clock signal that is synchronized with the received data. As such, the impacts of the transmission channel may be removed.